In the past, numerous different methods have been used for singulating or dicing a semiconductor wafer, the process of dividing a semiconductor wafer into individual devices. The two most widely used methods at this time are sawing using a diamond saw blade and laser scribing using a focused laser beam to cut through the wafer. Neither method is ideal. Both result in a significant loss of material during the cutting process. As the size of semiconductor devices get smaller, the width of the line of lost material during the dicing process becomes comparable to the width of the device. If the width of the line of material lost during the dicing process could be made significantly smaller, many more devices could be made on each wafer, resulting in a large savings in the cost of fabricating the devices. In addition, both the sawing and laser scribing are processes that are done one line or a few lines at a time, resulting in a lengthy process time to complete the singulation of all devices on each semiconductor wafer.
Since the invention of plasma and reactive ion etching in the 1970s, many individuals have proposed using these processes for wafer singulation. These processes potentially could decrease the material loss during the dicing process by etching very narrow lines through the semiconductor wafer between the individual devices. In addition, since all of the lines on the semiconductor wafer are etched through at the same time, the length of the time required to complete the process can be significantly shorter than the other processes. In order for a plasma etching or a reactive ion etching process to be effective in wafer dicing, it would have to etch very deep, narrow trenches in the streets of the semiconductor wafer and it would have to etch at a very fast etch rate to be economically attractive. These two conditions have been achieved in the last several years by several companies, building on the work of Laermer, et al., (U.S. Pat. Nos. 5,501,893, 6,531,068 and 6,284,148). This method of using gas plasma to etch deep, narrow features is known as Deep Reactive Ion Etching (DRIB) or the Bosch process. The DRIB or Bosch process is used for high aspect ratio etching by alternating deposition and etching cycles. The deposition of a polymer layer protects the side walls from chemical etching during the subsequent etching cycle. Directional etching caused by ion bombardment removes the polymer layer at the bottom of the etched feature, so that the gas radials created by the plasma are able to etch the semiconductor substrate.
Semiconductor wafers usually have one or more metal layers applied to the back of the wafer during fabrication to provide ohmic contact and/or ease of die attach during packaging of the devices. These layers of metal are not readily etched using plasma etch processes. For the plasma dicing process to be economical, a cost effective method of removing the back metal that remains in the streets after the etch process is completed must be employed. Lindsey (U.S. Pat. No. 8,450,188) describes a method of removing the back metal layer by flipping the wafer over on another piece of adhesive plastic file, after the plasma etch has been completed and cutting the back metal. Burghout, et al., (U.S. Pat. No. 8,664,089) describes a method of cutting the back metal using a fluid ejected from a nozzle. Lindsey, et al., (U.S. Pat. No. 8,906,745) describes a method of separating the back metal in the streets by applying pressure using a fluid. Grivna (U.S. Pat. No. 9,136,173) describes a method of separating the back metal by applying a localized pressure using a stylus. The current invention teaches a method of modifying the plasma dicing process to improve the cleaving of the back metal along the edges of each die.